I’m no GUI designer, it’d look like a childs scrawl!
In a nutshell I think that the way ableton does it works very well. You can essentially nest a set of FX in serial inside a wrapper. No modular madness, just a set of devices in serial that are wrapped in a parent device. This way you can nest wrappers within wrappers, so the flexibility is great but the signal and GUI flow is still quite simple.
If you think about a select box (much like the list of FX in renoise) as a way to select the current chain to display, then the GUI remains simple. You can only ever see a single chain within any wrapper at one time, so the the single horizontal list of FX paradigm remains. So you could easily split a single in two, run it through two chains of FX, and then merge it back into a single single for further processing afterwards.
Ableton also throws in the concept of “macro controls” which are basically like a hydra device, they allow control of multiple parameters with a single control. These two concepts combined (nestable chains and hydra control) makes for a very flexible and powerful way to use FX.